1. Field of the Invention
The present invention relates to a vehicle periphery monitoring device configured to monitor periphery of a vehicle on the basis of an image imaged by an imaging unit mounted in the vehicle.
2. Description of the Related Art
Hitherto, there has been disclosed a vehicle periphery monitoring device provided with a stereo camera, namely two cameras mounted with optical axes thereof parallel to each other, as a structure to calculate a parallax between an image section of an image of a target object imaged by the right camera and an image section of an image of the identical target object imaged by the left camera, calculate a distance from the vehicle to the target object by the usage of the parallax, and determine contact possibility between the target object and the vehicle on the basis of the calculated distance from the vehicle to the target object (for example, refer to Japanese Patent Laid-open No. 2001-6096).
For the identical target object, when the parallax between the first image section extracted from the image imaged by the right camera and the second image section extracted from the image imaged by the left camera is dx, the distance Z from the vehicle to the target object can be calculated according to the following relational expression (1).
                              [                      Relational            ⁢                                                  ⁢            expression            ⁢                                                  ⁢            1                    ]                ⁢                                                                                      Z        =                              f            p                    ·                      D            dx                                              (        1        )            
Wherein, Z: distance from the vehicle to the target object, f: focal length of camera, p: pixel pitch, D: baseline length of camera, and dx: parallax.
However, in practical, due to the following reasons, such as (a) effect of vibration from the vehicle in travelling, (b) aiming precision when mounting the cameras on the vehicle, (c) effect of correlation calculation when extracting the image sections of the identical target object, there exists an error between an actual distance from the vehicle to the target object (actual distance) and the distance calculated according to the above relational expression (1) (calculated distance).
As shown by the following relational expression (2), the error between the actual distance and the calculated distance affects the relational expression (1) mentioned above as a parallax offset α.
                              [                      Relational            ⁢                                                  ⁢            expression            ⁢                                                  ⁢            2                    ]                ⁢                                                                                      Z        =                              f            p                    ·                      D                          dx              +              α                                                          (        2        )            
Especially, as the distance Z from the vehicle to the target object becomes longer, the parallax dx becomes smaller, which makes the effect of the parallax offset α on the above relational expression (2) become remarkable. Thereby, the determination timing on the contact possibility between the target object and the vehicle by the usage of the distance would become irregular, which deteriorates the determination accuracy of the contact possibility.